The present invention relates to a process for purifying anhydrous hydrogen chloride gas (aHCl), preferably the anhydrous hydrogen chloride gas recovered from an isocyanate production process and to equipment suitable for use in this process. In the process of the present invention, the content of chlor-aromatics may be reduced from up to 1000 ppm to below 10 ppm levels to make the treated hydrogen chloride gas usable in a catalytic oxychlorination process.
A number of important chemical processes generate anhydrous hydrogen chloride (aHCl) as a byproduct. Examples of such processes include chlorination processes, silane production processes and phosgenation processes. Because large amounts of aHCl can not be disposed of, one of the challenges encountered with each of these processes is purification of the aHCl generated to obtain a usable technical product or raw material for other processes. Several processes for purifying aHCl generated during production processes have been proposed. Thermal treatment of the aHCl at temperatures of up to 800-1600xc2x0 C. is disclosed in U.S. Pat. No. 5,126,119. Full condensation and distillation under elevated pressure is disclosed in U.S. Pat. No. 4,935,220. However, these processes require high amounts of energy and critical, expensive equipment.
In the commercial phosgenation processes for the production of isocyanates such as TDI (toluene diisocyanate), MDI (diphenylmethane diisocyanates), and HDI (hexamethylene diisocyanate), two moles of aHCl are formed per isocyanate group produced. This large quantity of byproduct must be used in a secondary process.
One such secondary process is the production of muriatic acid. Another alternative is to use the aHCl in a catalytic oxychlorination process with ethylene to produce ethylene dichloride and finally vinyl chloride as the commercial product. This catalytic process is very sensitive to traces of organic compounds, particularly (chloro-) aromatic compounds which can deactivate the catalyst employed.
The most commonly used solvents in isocyanate production are chlorobenzene and dichlorobenzene. (See G. Oertel, Polyurethane Handbook, page 66 (Carl Hanser Verlag, Munich (1985)). The aHCl recovered from the phosgenation process is saturated with these chloroaromatics. Deep chilling of the aHCl gas can reduce the chloroaromatics content, but not to the necessary level. Another complicating factor is the high melting point of dichlorobenzene (o-isomer: xe2x88x9217.5xc2x0 C., p-isomer: +52.8xc2x0 C.), which limits the usefulness of this approach. More specifically, low pressure phosgenation processes such as those described in G. Oertel, Polyurethane Handbook, p.66 (Carl Hanser Verlag, Munich (1985)) which yield aHCl gas at pressures ranging from atmospheric to below 5 bar overpressure will, even with deep chilling, contain chloroaromatics in a concentration of from several hundred ppm to 1000 ppm.
It is an object of the present invention to provide a process and equipment useful for removing one or more contaminants from hydrogen chloride gas.
It is also an object of the present invention to provide a process and equipment useful for separating small quantities of high boiling material, e.g., (chloro) aromatic compounds from large volumes of anhydrous HCl gas.
It is another object of the present invention to provide a process for reducing the concentration of contaminants such as (chloro)aromatic compounds in anhydrous HCl gas to  less than 10 ppm.
These and other objects which will be apparent to those skilled in the art are accomplished by compressing anhydrous hydrogen chloride gas containing a (chloro)aromatic compound, cooling the compressed gas to reduce the temperature of the gas to 5-20xc2x0 C. above the outlet temperature of the process in the second stage but above the freezing point of the highest melting compound in a first cooling stage, further cooling the gas in a second cooling stage to reduce the temperature of the gas to at least xe2x88x9220xc2x0 C., returning the cooled gas from the second cooling stage to the first cooling stage, and recovering condensate streams which contain (chloro)aromatic compound from the first and second cooling stages.